Method of producing filamentary monocrystals

ABSTRACT

Described is an improvement in the method of producing filamentary monocrystals by precipitation of crystal substance from a vaporous compound in a reaction vessel at elevated temperature, which comprises supplying the reaction space in the vessel with a mist of droplets formed of seed material for the monocrystals to be produced, said droplet material having a melting point 50* to 500* C. below that of the monocrystals to be produced; maintaining said reaction space at a temperature between the respective melting points of said droplet material and said monocrystals; and contacting the vaporous compound in said reaction space with said mist of droplets, whereby filamentary monocrystals grow onto the droplets. Also described is apparatus for performing the method.

United States Patent [72] Inventor Julius Nick] Neulteferloh nearMunich, Germany [21] Appl. No. 9,114 [22] Filed Feb. 9, 1970 [45]Patented Sept. 21, 1971 [73] Assignee Siemens AktiengesellschaftContinuation of application Ser. No. 548,014, May 5, 1966, nowabandoned.

[54] METHOD OF PRODUCING FILAMENTARY MONOCRYSTALS 6 Claims, 2 DrawingFigs.

[52] U.S. Cl 23/223.5, 23/1, 23/183, 23/209, 148/1.6, 106/55 [51] Int.Cl. .lC0lb 33/02 [50] FieldofSear-ch 23/1, 138, 139,l83,140.147.148.209,223.5;148/1.6

[56] References Cited UNITED STATES PATENTS 2,904,404 9/1959 Ellis, Jr23/223.5 3,125,416 3/1964 Ryshkewitch et a1 23/183 3,129,059 4/1964 Enket a1 23/1 X Primary Examiner-Edward Stern Attorney-Curt M1 AveryABSTRACT: Described is an improvement in the method of producingfilamentary monocrystals by precipitation of crystal substance from avaporous compound in a reaction vessel at elevated temperature, whichcomprises supplying the reaction space in the vessel with a mist ofdroplets formed of seed material for the monocrystals to be produced,said droplet material having a melting point 50 to 500 C. below that ofthe monocrystals to be produced; maintaining said reaction space at atemperature between the respective melting points of said dropletmaterial and said monocrystals; and contacting the vaporous compound insaid reaction space with said mist of droplets, whereby filamentarymonocrystals grow onto the droplets. Also described is apparatus forperforming the method.

PATENTEDSEP21 [97:

METHOD OF PRODUCING FILAMENTARY MONOCRYSTALS his is a continuation ofapplication Ser. No. 548,014, filed May 5, 1966, now abandoned.

This invention relates to a method of producing filamentary monocrystalssuch as those commonly designated as whiskers." On account of theirextreme, nearly theoretical mechanical strength (kglmm which may exceedby a multiple that of strongest structural steel, such whiskers areexpected to afford a significant improvement of work materials analogousto the reinforcement of synthetic plastics by embedment of glass fibers.

Several methods are known for producing whiskers from many differentmaterials. Reference in this respect may be had, for example, to thebooks by Doremus, Roberts and Turnbull: Growth and Perfection ofCrystals (Wiley and Son, i958, pages 44-54), and Wilke: Methoden derKristallzuchtung (1963, pages 436-438, 455-460). It is known from thesebooks that whiskers can be made by dissociation of a vapor with the aidof a chemical reaction. The known methods of producing whiskers,however, furnish only individual specimens, so that it has beeninfeasible to industrially utilize the theoretically valuable propertiesof whiskers on a commercial scale.

It is an object of my invention to devise a method which affords acontinuous production of whiskers and which lends itself for economicaluse in industry.

To this end, the invention is predicated upon the production offilamentary monocrystals by thermal or chemical dissociation of avaporous chemical compound of the whisker substance or of a componentthereof. The chemical compound preferably forms part of a reaction gasmixture and is subjected to the dissociating reaction in a reactionvessel at elevated temperature. According to my invention l supply thereaction vessel not only with the vaporous reaction gas to bedissociated but also with a supply of crystal-seed substance.Preferably, the seed substance is identical with the compound to bedissociated but is given an addition of impurities in such a quantity asto reduce the melting point 50 to 500 C. below that of the puresubstance. 1 further heat the reaction space proper of the vessel to atemperature between the melting point of the impurity-containingsubstance and the melting point of the pure substance, convert theimpurity-containing substance to a mist of molten droplets, and contactthe reaction vapor with the mist within the heated space of the reactionvessel.

The conversion of the impure seed substance to a mist of fine dropletscan be effected within the reaction vessel by means of a plasma burnerfed, for example, with hydrogen and argon or other noble gas, or bymeans of an electrical arc device.

According to another feature of the invention, the vaporous reactionmixture is supplied at one end of an elongated reaction vessel, and theimpure seed material is dispersed into droplets at the same end. Theexcess amount or residue of the vaporous reaction gas mixture is takenfrom the other end of the reaction vessel and may be recycled back tothe first-mentioned end of the vessel. The whiskers then grow in theheated reaction space from the substance precipitating out of thevaporous reaction mixture onto the individual droplets and can becontinuously collected in a container located at the vessel end oppositethe location of formation.

According to further features of the invention, the elongated reactionvessel is provided with a jacket-type heater for heating it to thetemperature between the melting point of the impurity-containing dropletmaterial and the melting temperature of the pure material of thewhiskers to be grown. At the entering end of the reaction mixture thereis provided a dispersing or atomizing device such as the above-mentionedplasma burner or are device which produces the mist of liquid dropletsfrom material supplied for example in form of a rod. The opposite end ofthe reaction vessel may be directly designed as a collecting containerfor the continuously resulting whiskers. Also located at thefirst-mentioned end of the reaction vessel is a device for supplying thevaporous reaction mixture which contains chemically bound whiskermaterial, preferably together with hydrogen or other reactive or diluentgas. The residue that does not participate in the reaction issues froman outlet at the opposite end of the reaction vessel. For economy it isfurther advisable to provide a connecting eonduit between the inlet andoutlet of the reaction vessel so that the reaction mixture iscontinuously recirculated, aside from being continuously replenished ina controlled manner.

The invention takes advantage of the phenomenon that a vaporoussubstance will condense at droplets of crystallographically the same orsimilar material acting as liquid condensation nuclei. While generallythe substance thus condensed is likewise liquid so that solid filamentsor whiskers cannot normally be produced in this manner, the inventionresorts to an artifice to nevertheless achieve that the materialcondensing at the droplets will be solid. This is why the inventionrequires using condensation nuclei of a more or less contaminatedsubstance so that their melting point is lowered, whereas the condensingsubstance-by comparisonis pure because it is chemically or thermallyliberated from a chemical compound. Since under such conditions thereaction vessel is kept at a temperature between the respective meltingpoints of the contaminated substance and the relatively pure substanceto be condensed, the pure substance, condensing at the liquid droplets,converts to the solid constitution of the whiskers.

The condensing substance does not grow randomly onto the droplets butrather prefers as point of origin any pure and solid particles alreadycondensed at one or more points of a droplet. These starting localitiesof the growth are to be looked upon as constituting crystal seeds fromwhich the monocrystals will grow in a manner comparable to thephenomenon observed with the known epitaxial process. This peculiarobservation of singular starting points at minuscule droplets of a mistcan be explained for example as follows. As a rule, monocrystals grow ina crystallographically favorable direction if all other conditions arethe same on all sides of the crystal seed. Since the monocrystallinegrowth commences at a virtually pointlikc crystal seed and the otherconditions in the reaction vessel are everywhere the same, the crystalwill grow as a filament or whisker.

The method is applicable to all substances satisfying the fundamentalprocessing conditions. For example, whiskers can thus be produced fromsemiconductor materials such as silicon, germanium and boron, frommetals such as iron, and from oxides, particularly metal oxides such asberyllium oxide. General rules for the selection of substances suitablefor the method of the invention result from the following conditions:

a. In the first place, the method of the invention is applicable only tosubstances that can be liquefied under economically acceptableconditions. It is not feasible if no liquid droplets of the whiskermaterial can be produced, as is the ease with graphite, for example.

b. By method of the invention there can be produced whiskers of onlysuch materials which form a chemical compound or reaction mixture whoseboiling point is below the melting temperature of the pure substance, orwhich at least possesses such a high vapor pressure that the reactiongas mixture will entrain a sufficient amount of the whisker material.This condition is predicated upon the fact that the reaction gasmixture, containing the whisker material in chemically bound condition,is to be gaseous or vaporous in the same space of the reaction vesseland consequently at one and the same temperature at which the purewhisker material is solid.

c. Furthermore, the reaction gas mixture (or one of its components inwhich the whisker material is chemically bound) must be reducible ordisproportionable at temperatures below the melting point of the purewhisker material. Only if this condition is met, will whisker materialbe liberated in the reaction space so that whiskers will be formed.

Generally the vaporous reaction gas mixture, containing the material forthe whiskers, is supplied in form of a compound, for example ahalogenide. Preferably used is a chloride or iodide mixed with hydrogen.In the heated reaction space of the vessel there occurs adisproportioning or reduction, for example a hydrogen reduction, of thereaction gas mixture so that solid atomic or molecular whisker materialbecomes segregated and precipitates predominantly at the droplets in theform of monocrystalline whiskers. Since at the reaction temperature theimpure droplet material is maintained in liquid condition, it can act asa catalyst, for example, for the reduction of the above-mentionedhalogenide, aside from serving as a crystal seed for whisker formation.

In most cases several whiskers of respectively different lengths anddifferent diameters will grow on a single droplet of the mist. However,it also happens that only one whisker comes about at a single droplet.Only rarely has it been observed that droplets are located at both endsof a single whisker. The diameter of the whiskers made by the methodaccording to the invention is in the range of about 0.1 to about 100micron, and their length is between about 1 and 30 mm.

The method of the invention will be further elucidated by an examplerelating to the production of silicon whiskers and with reference to theaccompanying drawing in which:

FIG. 1 shows schematically an apparatus for continuously producingwhiskers; and

FIG. 2 shows schematically an electric arcing device for dispersing theimpure droplet material in production apparatus otherwise correspondingto FIG. ll.

In the apparatus shown in FIG. I, the impure droplet material issupplied in form ofa thin rod I consisting of silicon and having, forexample, a diameter of 3 mm. Due to the impurity contained in thesilicon, the melting point of the rod material is reduced 50 to 500 C.below the melting point of the pure silicon as it precipitates from thereaction gas used in the apparatus (1250 to 1420C). For example, a rod 1of contaminated silicon having a melting point at about 1000 C. issuitable. The rod 1 is passed into the reaction vessel 3 through aplasma burner 2 which operates to atomize the silicon into a mist ofliquid droplets. The silicon mist enters into contact with the vaporousreaction mixture supplied into the reaction vessel 3 through a feederdevice 5 to pass longitudinally through the vessel in the directionindicated by an arrow 7.

A heater jacket 4 maintains the interior of the reaction vessel at atemperature between 900 and I 100 C., this temperature being above themelting point of the impure silicon of rod 1 but below the meltingtemperature ofpure silicon. The reaction gas mixture consists, forexample, of about 6 percent by volume of silicon tetrachloride (SiCl theremainder being hydrogen. This mixture, when heated to 900-l l00 C.,causes reduction of silicon tetrachloride by hydrogen and evolution ofsolid silicon. The reaction takes place predominantly at the droplets ofthe silicon mist so that the droplets continuously receive theprecipitating silicon. The solid silicon then results in the growth ofwhiskers in the above-described manner. The rate of growth in theembodiment just described is in the order from millimeters to meters persecond. The resulting whiskers drop in the direction of the arrow 7 intoa collecting container 8 from which they can be taken from time to time.The remaining reaction gases leave the reaction vessel 3 through anoutlet collector 7 and can be recycled-entirely or partlyinto thereaction vessel through a line 9.

The method is analogously applicable for producing whiskers from metalsand oxides. In this case, the semiconductor rod 1 is substituted by thegreatly contaminated metal which is atomized into a mist of dropletswithin the reaction vessel, and the above-mentioned silicontetrachloride or other semiconductor halogenide in the reaction gasmixture is substituted by a halogenide of the metal from which thewhiskers are to be grown. For the production of oxide whiskers, the

vaporous reaction gas mixture may consist, for example, of oxygen andthe halogenicle of the particular metal contained in the oxide.

In lieu of the plasma burner (2 in FIG. 1), the mist ofliquid It will beunderstood that this device then substitutes the top portion of thevessel containing the plasma burner 2 and the rod 1 in FIG. 1. The are10 is maintained between electrode rods 11 and 12 supplied with currentfrom a suitable source 15 (are 10). In the illustrated example, hydrogenis supplied through an inlet opening 13 into the interior of the vesselin the direction indicated by an arrow 14. At the approximate localityof the arrow 14, the flow of hydrogen becomes laden with the droplets ofthe mist. These are produced by the are 10 from the material of theelectrode rods 11 and 12 consisting of the purposely impure metal orsemiconductor material. An arc-type dispersing device as shown in FIG. 2is well suitable for the production of metal whiskers or semiconductorwhiskers.

Whiskers produced by the method of the invention are applicable, interalia, for the production of composite or bonded materials together witha matrix of thermoplastic or metal, particularly aluminum, copper ormagnesium, thus resulting in reinforced heterogeneous materials ofimproved mechanical, electrical or other physical properties.

Iclaim:

I. In the method of producing filamentary monocrystals by precipitationof crystal substance from a vaporous compound in a reaction vessel atelevated temperature, the improvement which comprises supplying thereaction space in the vessel with a mist of droplets of the samematerial serving as seed crystals to be produced, said droplet materialserving as seed crystals for the monocrystals to be produced, addingimpurity to the droplet forming substance for reducing its melting point50 to 500 C. below that of the monocrystals to be produced; maintainingsaid reaction space at a temperature between the respective meltingpoints of said droplet material and said monocrystals; contacting thevaporous compound in said reaction space with said mist of droplets,whereby filamentary monocrystals grow onto the droplets; said crystalsubstance being silicon and said droplet material being relativelyimpure silicon material, and said compound consisting of a halogenide ofsaid semiconductor material and being mixed with hydrogen.

2. The method according to claim 1, which comprises supplying saiddroplet material in coherent constitution into the reaction vessel, anddispersing it inside the vessel to thereby form said mist of droplets insaid reaction space.

3. The method according to claim 1, which comprises continuouslysupplying said droplet material in coherent constitution into the vesseland dispersing it inside the vessel into said mist of droplets, passingsaid vaporous compound in a continuous flow through said vessel from aninlet point near the dispersing locality of said droplet material so asto entrain the droplets and the filamentary monocrystals, and collectingthe monocrystals at a vessel locality spaced in the entrainmentdirection from said dispersing locality.

4. The method according to claim 1, which comprises sup plying saiddroplet material and said vaporous compound near the top of said vessel,and collecting the filamentary monocrystals at the bottom of saidvessel.

5. The method according to claim I, which comprises dispersing saiddroplet material inside said vessel by feeding it through a plasmaburner.

6. The method according to claim 5, which comprises operating the plasmaburner with a mixture of hydrogen and noble gas.

2. The method according to claim 1, which comprises supplying saiddroplet material in coherent constitution into the reaction vessel, anddispersing it inside the vessel to thereby form said mist of droplets insaid reaction space.
 3. The method according to claim 1, which comprisescontinuously supplying said droplet material in coherent constitutioninto the vessel and dispersing it inside the vessel into said mist ofdroplets, passing said vaporous compound in a continuous flow throughsaid vessel from an inlet point near the dispersing locality of saiddroplet material so as to entrain the droplets and the filamentarymonocrystals, and collecting the monocrystals at a vessel localityspaced in the entrainment direction from said dispersing locality. 4.The method according to claim 1, which comprises supplying said dropletmaterial and said vaporous compound near the top of said vessel, andcollecting the filamentary monocrystals at the bottom of said vessel. 5.The method according to claim 1, which comprises dispersing said dropletmaterial inside said vessel by feeding it through a plasma burner. 6.The method according to claim 5, which comprises operating the plasmaburner with a mixture of hydrogen and noble gas.